Internet Engineering Task Force (IETF) C. Holmberg
Request for Comments: 7345 I. Sedlacek
Category: Standards Track Ericsson
ISSN: 2070-1721 G. Salgueiro
Cisco
August 2014
UDP Transport Layer (UDPTL)over Datagram Transport Layer Security (DTLS)
Abstract
This document specifies how the UDP Transport Layer (UDPTL) protocol,
the predominant transport protocol for T.38 fax, can be transported
over the Datagram Transport Layer Security (DTLS) protocol, how the
usage of UDPTL over DTLS is indicated in the Session Description
Protocol (SDP), and how UDPTL over DTLS is negotiated in a session
established using the Session Initiation Protocol (SIP).
Status of This Memo
This is an Internet Standards Track document.
This document is a product of the Internet Engineering Task Force
(IETF). It represents the consensus of the IETF community. It has
received public review and has been approved for publication by the
Internet Engineering Steering Group (IESG). Further information on
Internet Standards is available in Section 2 of RFC 5741.
Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
http://www.rfc-editor.org/info/rfc7345.
Copyright Notice
Copyright (c) 2014 IETF Trust and the persons identified as the
document authors. All rights reserved.
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the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.
Holmberg, et al. Standards Track [Page 1]

RFC 7345 UDPTL over DTLS August 20141. Introduction
While it is possible to transmit highly sensitive documents using
traditional telephony encryption devices, secure fax on the Public
Switched Telephone Network (PSTN) was never widely considered or
prioritized. This was mainly because of the challenges involved with
malevolent physical access to telephony equipment. As real-time
communications transition to IP networks, where information might
potentially be intercepted or spoofed, an appropriate level of
security for fax that offers integrity and confidentiality protection
is vital.
The overwhelmingly predominant fax transport protocol is UDPTL-based,
as described in Section 9.1 of [ITU.T38.2010]. The protocol stack
for fax transport using UDPTL is shown in Figure 1.
+-----------------------------+
| Internet facsimile protocol |
+-----------------------------+
| UDPTL |
+-----------------------------+
| UDP |
+-----------------------------+
| IP |
+-----------------------------+
Figure 1: Protocol Stack for UDPTL over UDP
The following mechanisms are available for securing fax:
o Annex H of [ITU.T30.2005] specifies an application-layer integrity
and confidentiality protection of fax that is independent of the
transport protocol and is based on the RSA algorithm for use with
the T.30 telephony protocol by Group 3 facsimile equipment (G3FE).
o [ITU.T38.2010] specifies fax transport over RTP/SAVP, which
enables integrity and confidentiality protection of fax in IP
networks.
Both of these mechanisms have been available for many years and never
gained any significant adoption in the market. This has prompted an
effort to develop an approach, based on open standards, for securing
fax communications over an IP-based transport.
Telephony-based protocols like T.30 offer application-level security
options like the RSA-based approach detailed in Annex H of the T.30
specification [ITU.T30.2005]. The problem is that it is very
sparingly implemented and not enforced at the transport level.
Holmberg, et al. Standards Track [Page 3]

RFC 7345 UDPTL over DTLS August 2014
It is worth noting that while T.38 over RTP offers a very viable
option for such standards-based IP security solution using Secure
Realtime Transport Protocol (SRTP), this fax-over-IP transport never
gained any traction in the marketplace and accounts for a negligible
percentage of fax-over-IP implementations.
Thus, security mechanisms offering integrity and confidentiality
protection should be limited to UDPTL-based fax transport, which is
the only broad-based fax-over-IP solution. The 3rd Generation
Partnership Project (3GPP) launched a study on how best to provide
secure fax in the IP Multimedia Subsystem (IMS) for UDPTL. Results
of the study confirmed that this security was best achieved by using
UDPTL over DTLS.
This document specifies fax transport using UDPTL over DTLS
[RFC6347], which enables integrity and confidentiality protection of
fax in IP networks. The protocol stack that enhances fax transport
to offer integrity and confidentiality using UDPTL over DTLS is shown
in Figure 2.
+-----------------------------+
| Internet facsimile protocol |
+-----------------------------+
| UDPTL |
+-----------------------------+
| DTLS |
+-----------------------------+
| UDP |
+-----------------------------+
| IP |
+-----------------------------+
Figure 2: Protocol Stack for UDPTL over DTLS over UDP
The primary motivations for the mechanism in this document are:
o The design of DTLS [RFC6347] is clearly defined and well
understood, and implementations are widely available.
o No DTLS extensions are required in order to enable UDPTL transport
over DTLS.
o Fax transport using UDPTL over DTLS only requires insertion of the
DTLS layer between the UDPTL layer and the UDP layer, as shown in
Figure 2. The UDPTL layer and the layers above the UDPTL layer
require no modifications.
Holmberg, et al. Standards Track [Page 4]

RFC 7345 UDPTL over DTLS August 2014
o UDPTL [ITU.T38.2010] is by far the most widely deployed fax
transport protocol in IP networks.
o 3GPP and the IP fax community need a mechanism to transport UDPTL
over DTLS in order to provide secure fax in SIP-based networks
(including IMS).
This document specifies the transport of UDPTL over DTLS using the
DTLS record layer "application_data" packets [RFC5246] [RFC6347].
Since the DTLS record layer "application_data" packet does not
indicate whether it carries UDPTL or some other protocol, the usage
of a dedicated DTLS association for transport of UDPTL needs to be
negotiated, e.g., using the Session Description Protocol (SDP)
[RFC4566] and the SDP offer/answer mechanism [RFC3264].
Therefore, this document specifies a new <proto> value [RFC4566] for
the SDP media description ("m=" line) [RFC3264], in order to indicate
UDPTL over DTLS in SDP messages [RFC4566].
2. Conventions
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in BCP 14, RFC 2119
[RFC2119].
DTLS uses the term "session" to refer to a long-lived set of keying
material that spans DTLS associations. In this document, in order to
be consistent with SIP/SDP usage of "session" terminology, we use
"session" to refer to a multimedia session and use the term "DTLS
session" to refer to the DTLS construct. We use the term "DTLS
association" to refer to a particular DTLS cipher suite and keying
material set that is associated with a single host/port quartet. The
same DTLS session can be used to establish the keying material for
multiple DTLS associations. For consistency with other SIP/SDP
usage, we use the term "connection" when what's being referred to is
a multimedia stream that is not specifically DTLS.
3. Secure Channel
The UDPTL-over-DTLS media stream is negotiated using the SDP offer/
answer mechanism [RFC3264]. See Section 4 for more details.
DTLS is used as specified in [RFC6347]. Once the DTLS handshake is
successfully completed (in order to prevent facsimile data from being
transmitted insecurely), the UDPTL packets MUST be transported in
DTLS record layer "application_data" packets.
Holmberg, et al. Standards Track [Page 5]

RFC 7345 UDPTL over DTLS August 20144. SDP Offerer/Answerer Procedures4.1. General
An endpoint (i.e., both the offerer and the answerer) MUST create an
SDP media description ("m=" line) for each UDPTL-over-DTLS media
stream and MUST assign a UDP/TLS/UDPTL value (see Table 1) to the
"proto" field of the "m=" line.
The procedures in this section apply to an "m=" line associated with
a UDPTL-over-DTLS media stream.
In order to negotiate a UDPTL-over-DTLS media stream, the following
SDP attributes are used:
o The SDP attributes defined for UDPTL over UDP, as described in
[ITU.T38.2010]; and
o The SDP attributes, defined in [RFC4145] and [RFC4572], as
described in this section.
The endpoint MUST NOT use the SDP "connection" attribute [RFC4145].
In order to negotiate the TLS roles for the UDPTL-over-DTLS transport
connection, the endpoint MUST use the SDP "setup" attribute
[RFC4145].
If the endpoint supports, and is willing to use, a cipher suite with
an associated certificate, the endpoint MUST include an SDP
"fingerprint" attribute [RFC4572]. The endpoint MUST support SHA-256
for generating and verifying the SDP "fingerprint" attribute value.
The use of SHA-256 is preferred. UDPTL over DTLS, at a minimum, MUST
support TLS_DHE_RSA_WITH_AES_128_GCM_SHA256 and MUST support
TLS_ECDHE_RSA_WITH_AES_128_GCM_SHA256. UDPTL over DTLS MUST prefer
TLS_ECDHE_RSA_WITH_AES_128_GCM_SHA256 and any other Perfect Forward
Secrecy (PFS) cipher suites over non-PFS cipher suites.
Implementations SHOULD disable TLS-level compression.
If a cipher suite with an associated certificate is selected during
the DTLS handshake, the certificate received during the DTLS
handshake MUST match the fingerprint received in the SDP
"fingerprint" attribute. If the fingerprint does not match the
hashed certificate, then the endpoint MUST tear down the media
session immediately. Note that it is permissible to wait until the
other side's fingerprint has been received before establishing the
connection; however, this may have undesirable latency effects.
Holmberg, et al. Standards Track [Page 6]

RFC 7345 UDPTL over DTLS August 20144.2. Generating the Initial Offer
The offerer SHOULD assign the SDP "setup" attribute with a value of
"actpass", unless the offerer insists on being either the sender or
receiver of the DTLS ClientHello message, in which case the offerer
can use either a value of "active" (the offerer will be the sender of
ClientHello) or "passive" (the offerer will be the receiver of
ClientHello). The offerer MUST NOT assign an SDP "setup" attribute
with a "holdconn" value.
If the offerer assigns the SDP "setup" attribute with a value of
"actpass" or "passive", the offerer MUST be prepared to receive a
DTLS ClientHello message before it receives the SDP answer.
4.3. Generating the Answer
If the answerer accepts the offered UDPTL-over-DTLS transport
connection, in the associated SDP answer, the answerer MUST assign an
SDP "setup" attribute with a value of either "active" or "passive",
according to the procedures in [RFC4145]. The answerer MUST NOT
assign an SDP "setup" attribute with a value of "holdconn".
If the answerer assigns an SDP "setup" attribute with a value of
"active" value, the answerer MUST initiate a DTLS handshake by
sending a DTLS ClientHello message on the negotiated media stream,
towards the IP address and port of the offerer.
4.4. Offerer Processing of the Answer
When the offerer receives an SDP answer, if the offerer ends up being
active it MUST initiate a DTLS handshake by sending a DTLS
ClientHello message on the negotiated media stream, towards the IP
address and port of the answerer.
4.5. Modifying the Session
Once an offer/answer exchange has been completed, either endpoint MAY
send a new offer in order to modify the session. The endpoints can
reuse the existing DTLS association if the key fingerprint values and
transport parameters indicated by each endpoint are unchanged.
Otherwise, following the rules for the initial offer/answer exchange,
the endpoints can negotiate and create a new DTLS association and,
once created, delete the previous DTLS association, following the
same rules for the initial offer/answer exchange. Each endpoint
needs to be prepared to receive data on both the new and old DTLS
associations as long as both are alive.
Holmberg, et al. Standards Track [Page 7]

RFC 7345 UDPTL over DTLS August 20145. Miscellaneous Considerations5.1. Anonymous Calls
When making anonymous calls, a new self-signed certificate SHOULD be
used for each call, and attributes inside the certificate MUST NOT
contain information that allows either correlation or identification
of the user making anonymous calls. This is particularly important
for the "subjectAltName" and "commonName" attributes.
5.2. NAT Traversal5.2.1. ICE Usage
When Interactive Connectivity Establishment (ICE) [RFC5245] is being
used, the ICE connectivity checks are performed before the DTLS
handshake begins. Note that if aggressive nomination mode is used,
multiple candidate pairs may be marked valid before ICE finally
converges on a single candidate pair. User Agents (UAs) MUST treat
all ICE candidate pairs associated with a single component as part of
the same DTLS association. Thus, there will be only one DTLS
handshake even if there are multiple valid candidate pairs. Note
that this may mean adjusting the endpoint IP addresses if the
selected candidate pair shifts, just as if the DTLS packets were an
ordinary media stream. In the case of an ICE restart, the DTLS
handshake procedure is repeated, and a new DTLS association is
created. Once the DTLS handshake is completed and the new DTLS
association has been created, the previous DTLS association is
deleted.
5.2.2. STUN Interaction
The UA MUST send the Session Traversal Utilities for NAT (STUN)
packets [RFC5389] directly over UDP, not over DTLS.
The UA MUST support the following mechanism for demultiplexing
packets arriving on the IP address and port associated with the DTLS
association:
o If the value of the first byte of the packet is 0 or 1, then the
packet is STUN.
o If the value of the first byte of the packet is between 20 and 63
(inclusive), the packet is DTLS.
Holmberg, et al. Standards Track [Page 8]

RFC 7345 UDPTL over DTLS August 20145.3. Rekeying
During rekeying, packets protected by the previous set of keys can
arrive after the DTLS handshake caused by rekeying has completed,
because packets can be reordered on the wire. To compensate for this
fact, receivers MUST maintain both sets of keys for some time in
order to be able to decrypt and verify older packets. The duration
of maintaining the previous set of keys after the finish of the DTLS
handshake is out of the scope of this document.
5.4. Compatibility with UDPTL over UDP
If a user requires fax to be transported securely using UDPTL over
DTLS, and if the remote user does not support UDPTL over DTLS, then a
fax media stream cannot be established.
If a user prefers fax to be transported securely using UDPTL over
DTLS but is willing to transport the fax insecurely in case the
remote user does not support UDPTL over DTLS, then the SDP Capability
Negotiation mechanism [RFC5939] can be used to offer both UDPTL over
DTLS and UDPTL over UDP. Alternatively, if the remote user rejects
an SDP offer for UDPTL over DTLS, a new SDP offer for a UDPTL-over-
UDP media stream can be sent.
6. Security Considerations
Fax may be used to transmit a wide range of sensitive data, including
personal, corporate, and governmental information. It is therefore
critical to be able to protect against threats to the confidentiality
and integrity of the transmitted data.
The mechanism in this document provides integrity and confidentiality
protection for fax by specifying fax transport using UDPTL over DTLS
[RFC6347].
DTLS media stream negotiated using SIP/SDP requires a mechanism to
ensure that the certificate received via DTLS was issued by the
remote party of the SIP session.
The standard DTLS strategy for authenticating the communicating
parties is to give the server (and optionally the client) a PKIX
[RFC5280] certificate. The client then verifies the certificate and
checks that the name in the certificate matches the server's domain
name. This works because there are a relatively small number of
servers and the cost for issuing and deploying PKIX certificates can
be justified. Issuing and deploying PKIX certificates to all clients
is not realistic in most deployment scenarios.
Holmberg, et al. Standards Track [Page 9]

RFC 7345 UDPTL over DTLS August 2014
The design described in this document is intended to leverage the
integrity protection of the SIP signaling, while not requiring
confidentiality. As long as each side of the connection can verify
the integrity of the SDP received from the other side, then the DTLS
handshake cannot be hijacked via a man-in-the-middle attack. This
integrity protection is easily provided by the caller to the callee
via the SIP Identity [RFC4474] mechanism. Other mechanisms, such as
the S/MIME mechanism [RFC3261] or perhaps future mechanisms yet to be
specified, could also serve this purpose.
While this mechanism can still be used without such integrity
mechanisms, the security provided is limited to defense against
passive attack by intermediaries. An active attack on the signaling
plus an active attack on the media plane can allow an attacker to
attack the connection (R-SIG-MEDIA in the notation of [RFC5479]).
7. IANA Considerations
This document updates the "Session Description Protocol (SDP)
Parameters" registry as specified in Section 8.2.2 of [RFC4566].
Specifically, the values in Table 1 have been added to the SDP
"proto" field registry.
+-------+---------------+-----------+
| Type | SDP Name | Reference |
+-------+---------------+-----------+
| proto | UDP/TLS/UDPTL | [RFC7345] |
+-------+---------------+-----------+
Table 1: SDP "proto" Field Values
8. Acknowledgments
Special thanks to Peter Dawes, who provided comments on the initial
draft version of the document, and to Paul E. Jones, James Rafferty,
Albrecht Schwarz, Oscar Ohlsson, David Hanes, Adam Gensler, Ari
Keranen, Flemming Andreasen, John Mattsson, and Marc Petit-Huguenin,
who provided valuable feedback and input. Barry Leiba, Spencer
Dawkins, Pete Resnick, Kathleen Moriarty, and Stephen Farrell
provided valuable feedback during the IESG review. Thanks to Scott
Brim for performing the Gen-ART review. Thanks to Alissa Cooper for
her help as sponsoring Area Director.
Holmberg, et al. Standards Track [Page 10]

RFC 7345 UDPTL over DTLS August 2014Appendix A. ExamplesA.1. General
Prior to establishing the session, both Alice and Bob generate self-
signed certificates that are used for a single session or, more
likely, reused for multiple sessions.
The SIP signaling from Alice to her proxy is transported over TLS to
ensure an integrity-protected channel between Alice and her identity
service. Alice's identity service asserts identity of Alice and
protects the SIP message, e.g., using SIP Identity. Transport
between proxies should also be protected, e.g., by use of TLS.
In order to simplify the flow, only one element is shown for Alice's
and Bob's proxies.
For the sake of brevity and simplicity, only the mandatory SDP T.38
attributes are shown.
A.2. Basic Message Flow
Figure 3 shows an example message flow of session establishment for
T.38 fax securely transported using UDPTL over DTLS.
In this example flow, Alice acts as the passive endpoint of the DTLS
association, and Bob acts as the active endpoint of the DTLS
association.
Holmberg, et al. Standards Track [Page 13]

RFC 7345 UDPTL over DTLS August 2014
Message (3):
Assuming that Alice's identity is valid, Bob sends a DTLS
ClientHello directly to Alice.
Message (4):
Alice and Bob exchange further messages of DTLS handshake
(HelloVerifyRequest, ClientHello, ServerHello, Certificate,
ServerKeyExchange, CertificateRequest, ServerHelloDone,
Certificate, ClientKeyExchange, CertificateVerify,
ChangeCipherSpec, and Finished).
When Bob receives the certificate of Alice via DTLS, Bob checks
whether the certificate fingerprint calculated from Alice's
certificate received via DTLS matches the certificate fingerprint
received in the a=fingerprint SDP attribute of Figure 5. In this
message flow, the check is successful; thus, session setup
continues.
Note that, unlike in this example, it is not necessary to wait for
the DTLS handshake to finish before the SDP answer is sent. If
Bob has sent the SIP 200 (OK) response and later detects that the
certificate fingerprints do not match, he will terminate the
session.
Message (5):
Figure 6 shows a SIP 200 (OK) response to the initial SIP INVITE
request, sent by Bob to Bob's proxy. The SIP 200 (OK) response
contains an SDP answer.
The "m=" line in the SDP answer indicates T.38 fax using UDPTL
over DTLS.
The SDP "setup" attribute with a value of "active" in the SDP
answer indicates that Bob has requested to be the active endpoint.
The SDP "fingerprint" attribute in the SDP answer contains the
certificate fingerprint computed from Bob's self-signed
certificate.
Holmberg, et al. Standards Track [Page 17]